The PAPI (and VASI) system relies on the color variation of a multi-beam light to guide an aircraft to land with the correct glide slope. Due to the sensitive nature of landing operations, FAA has mandated regular testing and maintenance of PAPI systems. Among the FAA regulations and requirements are the following:                Order 6850—5c Maintenance of Lighted Navigational Aids        So_certalert—0204 VASI/PAPI Maintenance        PAPI Operational Safety Assessment        
The three most important parameters of the PAPI light beam to be checked are the following:
1. Light Beam Vertical Aiming Angle
If the angle is lowered from the set angle for some reason, landing aircraft descends below the glide path and may hit obstacle. If raised, the landing aircraft descends from the steeper angle and may have increased chance of accident.
2. Light Intensity
Light intensity determines whether the PAPI lights are visible at a distance of 3 miles by day and 20 miles by night, as required by FAA.
3. Transition Angle
FAA mandates a 3 minute transition zone separating the red and white light. The landing pilot will read ambiguous signal if the aircraft is flying at a height within the transition zone, thus increasing chances of error.
There are three more important parameters:
4. Light Intensity Distribution
While the current generation of PAPI uses incandescent light bulbs, the light intensity is evenly distributed, so taking one measurement works well. However, the next generation LED PAPI uses an array of LEDs as light source. The LEDs may fail individually and impact the particular area the PAPI light beam. Thus, it's more important to measure the light intensity distribution for the LED based PAPI systems.
5. Horizontal Tilt Angle
As required by FAA, the PAPI light beam covers a horizontal arc 10 degrees to the left of its central axis and 10 degrees to the right. Any horizontal tilt in the PAPI Light Housing Assembly (LHA) or its optical path, the height of red white transition line to the left of axis may differ from the height on the right side. The result is error in the peripheral glide path, even though the central axial glide path is correct.
6. Transition Angle Distribution
Similar to the horizontal tilt angle, due to imperfections in the optical system, the PAPI light beam red white transition angle on the ends of its horizontal arc may be more dispersed transition angle on the central axis.
The traditional testing methods include both ground-based and flight-based techniques, each with its own set of pros and cons.
1. In the basic test, the operator uses either a bubble or digital style aiming device to checks aiming angle of the LHA housing. Since the PAPI light beam is not directly measured, there may be error should the LHA case and light source or optical lens are misaligned. The PAPI light beam transition angle and light intensity is not measured.
2. In the more advanced test, the operator directly measures the PAPI light beam by mounting a theodolite on top of the LHA and erecting the staff 20 to 30 paces in front of the LHA. This test measures the true light beam optical aiming angle, however, because the observation is so close to the PAPI, the transition angle and light intensity distribution cannot be accurately measured.
In laboratory setting, the operator may measure the transition angle using the same setup, but take measurement much further, about 1000 feet away. However, this is unpractical for field testing.
3. The flight check gives accurate measurement of the PAPI light beam, as a pilot would use the system, but it's too costly for frequent testing cycles, such as monthly.
The proposed PAPI Field Testing Instrument (FTI) significantly improved upon the existing testing method. The FTI offers direct optical measurement of the PAPI light beam by collecting the beam and focusing it on a target plate. The light beam image on the target plate is the same as if viewed by the pilot from a distance. From the target plate, accurate and quantitative measurements are taken for the PAPI light beam vertical aiming angle, transition angle, light intensity distribution, and horizontal tilt angle. The PAPI FTI design and operation is described in the following sections.
The PAPI FTI is designed as a self contained package, easy to use, ruggedized, cost effective, and highly portable. In addition to regular airport maintenance, the FTI can also be used by the FAA Technical Center to certify new PAPI models and acceptance testing, and by PAPI manufactures for Quality Assurance test before shipment.